Spark gap modulator



J. R. PERKINS Dec. 11, 1951 SPARK GAP MODULATOR Filed Dec. 18, 1945 FIG.I

PULSE FORMING I NETWORM PULSE FORMING NETWORK PULSE FORMING NETWORK PULSE FORMING NETWORK INVEN TOR JOSEPH R. PERKINS ATTORNEY Patented Dec. 11, 1951 2,578,263 SPARK GAP MODULATOR Joseph R. Perkins, Upper Montclair, N. J., as-

signor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application December 1 8, 1945, Serial No. 635,790 7 Claims. (01. 171-97) 7 1 This invention relates to modulator circuits particularly those producing relatively high volt age rectangular pulses of short duration, such as are required to pulse a magnetron.

Magnetrons as used in present day electromagnetic object detectors and the like require a puls-1 ing voltage of rectangular wave form, having a relatively high intensity up to 50 kilovolts and a short duration of sometimes less than one micro second. To secure a rectangular waveform, pulse forming networks have been used, which comprise in their elementary form a capacitor for storing electrical energy together with a circuit for producing a wave of desired shape during the discharge of the capacitor. essary high voltage to pulse the magnetron the network must be charged to a relatively high volt- To. overcome this difiiculty a plurality of these net works may be connected to be effectively in parallel during charge and effectively in series during discharge. This permits a reduction in the voltage source, but requires some form of switching. means such as a plurality of spark gaps operative: in synchronism to produce a short circuit. To

age with a resulting high voltage source.

simultaneously trigger a plurality of spark gaps to produce a pulse of say one microsecond duration requires an accuracy within a fraction of a microsecond. This is accomplished in the instant invention by a circuit which practically instanta j neously produces a triggering of all spark gaps in response to the triggering ofone.

It is an object of this invention to provide a modulator circuit capable of producing a rectangular voltage wave of high intensity and short I duration froma relatively low voltage source.

It is a further object of this invention to provide a modulator circuit whose pulse forming networks are charged in parallel and discharged'in] series by a plurality of spark gaps operative si--.

Fig. 1 is generally a circuit diagram of the-- invention; and

Figs. 2 and 3 are details of certain elementsin Fig; 1.

'- Y In Fig. 1 is disclosed a modulator circuit which is used to pulse aload I0 which-may be ama'g To secure the nec- 7 netron from a source of pulsating or alternating voltage impressed on terminals II and I2. A transformer I3 is provided to step this voltage up to a value somewhat in excess of that to which it is desired to charge pulse forming networks I4, I5, I6 and IT to allow for losses. The voltage output from transformer I3 is passed through a half way rectifier I8, in the instant embodiment a diode, and impressed on pulse forming networks I4 through I1. Inductors 20 and 29 have each a magnitude of inductance such as to oiTer a low impedance which is essentially a short circuit at the frequency of the voltage impressed on terminals II' and I2. For a frequency of 400 cycles a value of 10 millihenries was found satisfactory. Charging paths may be traced from the cathode of diode I8 through network It and induo-tors 25 and 29. Similarly, paths may be traced from this cathode through inductor 20, network I5, and inductors 24, 28, and 29; and from this cathode through inductors 20, 2 I, network I6, and inductors 23, 21, 28, and 29; and so forth. With inductors offering a low impedance at the frequency of the I impressed voltage, networks I 4 through I! will be effectively in parallel across the output of rectifier I8, and during the charging half cycle will be each charged substantially to the peak voltage occurring there.

Between each of the networks [4 through I7 spark gaps 30, 3|, and 32 are connected in alternate manner therewith as shown. Also a spark gap 33 is connected between network I4 and ground. One of these gaps, namely 33, is disclosed in Fig. 2, and comprises two separate curved surfaces 39 and 40 to form the main elec trodes of the gap, and a trigger 38. If a voltage insuflicient to break down the gap is applied between the main electrodes at 39 and 40, the gap will hold. If, however, a discharge between trigger 38 and the electrode at 39 is caused by a trigger pulse the resulting ions will break down the gap between surfaces 39 and 40. An opening at 4| is provided through which a blast of air is forced to drive the ions to the space between surfaces 39 and 40.

Blocking condensers 34, 35, and 36 form a connection with the triggers of the respective gaps 30, 3I, and32 to ground, their purpose being to retard any voltage change at these triggers with respect to ground. Assume that the networks I 4 through I I have been charged and a trigger pulse is applied to terminal 37 to break down gap 33. This effectively grounds the end of network 1.4 adjacent gap 33 tomakethe end adlacentgap this gap. In like manner gap 32 is broken down.-

- Thus networks ll through I! are effectively connected by the switching action of the spark gaps in series with load to discharge their stored energy therethrough as a pulse depending on the electrical characteristics of the networks. The harmonic components of this pulse will have frequencies in the order of megacycles for a pulse of a microsecond duration. 'At these frequencies inductors through 29 have each a reactance sol high that they may be considered essentially 'ppencircuits, and the effects of any currents carried by-the inductors during discharge of the nfitworks may be ignored. Upon completion of the discharge'from the networks, the spark gap will extinguish-to permit recharging of the network from the source.

For continued operation the trigger pluse applied to terminal 31 will be synchronized with the frequency of the alternating voltage source applied at terminals H and I2 to fire the gaps preferably on the half cycle that rectifier I8 is non-conducting. A kicker may be employed for, this purpose, which may be any of many well known circuits which produces a sharp pulse at any desired time in each cycle of an alternating volta e. a r

.;; In Fig. 3 is disclosed a pulse forming network :for a one microsecond pulse-which may be used inthe; instant-invention. A suitable value for each condenser C is .0008 microfarad with the inductances L having each a value of about 20 microhenries. v

It is tobe understood that while four pulse forming networks with their associated circuits have been disclosed, some other number either greater or less may be used depending on the amount. of voltage increase from the transformer output desired.

The invention is only to be limited by the appended claims.

What isclaimed isz 1; A modulator circuit comprising, a plurality of pulse forming networks, a like plurality of spark gaps connected in serieswith said networks in alternate manner, a source of pulsating voltage, means eifectively connecting said net- 'work in parallel during the application of voltage from said source, a trigger for each of said spark gaps, and means for producing a voltage difference between each of said spark gaps less oneand its respective trigger, whereby when said one spark gap is triggered the'remaining spark gap's' will be also triggered to effectively connect said networks in series for discharge.

'es'sentially openfcircuits'at the frequency. of the components of-lthe'pulse formed byisaid network,

4 a trigger for each of said spark gaps, and means for retarding voltage change in each of the triggers of the respective gaps less one, whereby triggering said one gap will produce a voltage difference between each of the remainder of said gaps and its respective trigger to trigger the remainder of said gapsto effectivelyconnect said network in series. r

3. A modulator circuit comprising a source of alternating voltage, a half wave rectifier, and a spark gap all in series; a plurality of pulse formin networks, a spark gap connected between eachof said networks, and a load, all connected in seriesacross said first mentioned spark gap; a plurality of inductors of magnitudes such as to be essentially short circuits at the frequency of the alternating voltage and essentially an open circuit at the frequencies of the components of the pulse formed by said network, said inductors connecting said networks in parallel across said first-mentioned gap at the frequency or said alternating voltage to permit charging of said net- :works from said source through said rectifier; a trigger for said first mentioned gap synchronixed withthe alternating voltage to short circuit said first mentioned gap on the half wave corresponding to non-conduction of said rectifier; a trigger for each of the remainder of said spark gaps and including each a condenser for retarding voltage change .thereat, whereby triggering of said first mentioned gap will produce a voltage difierence between each of the remainder of said gaps and the respective triggers to trigger reinainder of said gaps to connect said networks in series with said load for discharge theresaid source, a trigger for each of said spark gaps,

means for producing a voltage difierence between each of said spark gaps less one and its respective trigger, and means for introducing quantities of ions into said spark gaps for facilitating breakdown of said spark gaps.

5. A modulator circuit comprising, a plurality of pulse forming networks, a resistive load, a plurality of spark gaps connected in series with said networks in alternate manner, means for charging said networks to'arelatively high value, a trigger electrode for each of said spark gaps, means for retarding voltage change between each of said gaps less one and its respective trigger electrode, and means for energizing said trigger electrodes .for causing serial discharge of said networks through said resistive load.

6. A modulator circuit comprising, a plurality of pulse forming networks, a plurality of spark gaps connected in series with said networks in alternate manner, a source of voltage, reactive means for effectively connecting said networks in the remainderof said gaps are fired-to effectively connect. s idn twork i eri s- H 7. A modulator circuit comprising, a plurality of pulse forming networks, a plurality of spark gaps connected in series therewith in alternate manner, a source of voltage, a plurality of inductors efifectively connecting said networks in parallel with said source, said inductors being of magnitude sufficient to be substantially short circuits at the frequency of said voltage source and substantially open circuits at the frequency of the components of pulses formed by said networks, a, trigger electrode for each of said spark gaps, and means for introducing an abundance of ions into said spark gaps for facilitating breakdown in response to energization of said trigger electrodes.

JOSEPH R. PERKINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 10 Number Name Date 1,997,064 Lusignan Apr. 9, 1935 2,405,069 Tonks July 30, 1946 2,408,824 Varela Oct. 8, 1946 

